In the past, there has been proposed a light emitting device utilizing a LED chip. This kind of the light emitting device generally employs a fluorescent material excited by light emitted from the LED chip and emitting light of a color different from a color of the light emitted from the LED chip in order to light having a desired color (for example, white) by means of color mixture of light.
For example, Japanese Patent Publication No. 3978451 discloses a light emitting device 101 shown in FIG. 10. The light emitting device 101 includes a LED chip 110, a mounting substrate 120 on which the LED chip 101 is mounted, a translucency encapsulation member 150, and a color conversion member 170. The translucency encapsulation member 150 is shaped into a semispherical shape and encapsulates the LED chip 110 on a surface of the mounting substrate 120 where the LED chip 110 is mounted. The translucency encapsulation member 150 is made of a first transparent material (e.g. silicone resin). The color conversion member 170 is shaped into a dome shape and is made of a second transparent material (e.g. silicone resin) containing a fluorescent material excited by light emitted from the LED chip 110 and emitting light longer in wavelength than the light emitted from the LED chip 110. The color conversion member 170 is secured to the mounting substrate 120 such that there is an air layer 180 interposed between the color conversion member 170 and the translucency encapsulation member 150. The above mentioned Japanese Patent Publication exemplifies a visible light LED chip together with an ultraviolet LED chip as the LED chip 110.
For example, the above mentioned light emitting device 101 is used for illumination purpose (e.g. the light emitting device 101 is used as a light source of a lighting fixture). In this instance, when an electrical current flowing through the light emitting device 101 is increased to obtain higher luminance, light or heat emitted from the fluorescent material is likely to cause time degradation to the second translucency material of the color conversion member 170, thereby reducing transmissivity of the second translucency material of the color conversion member 170. This is likely to cause some defect, such as, a defect where an amount of the light emitted from the light emitting device 101 is decreased, a defect where to lose a balance between the light emitted from the LED chip 110 and the light emitted from the fluorescent material causes a difference in chromaticity (color phase and chromaticness) between the light emitted from the LED chip 110 and the light emitted from the fluorescent material, and a defect where an operation life of the light emitting device 101 is shortened due to thermally deterioration of the color conversion member 170.
In view of above defects, it is considered to adopt not a translucency organic material (e.g. silicone resin and the like) but a translucency inorganic material (e.g. glass and the like) as the second translucency material of the color conversion layer 170. The translucency inorganic material can radiate the heat generated by the fluorescent material efficiently in comparison with the translucency organic material, because the translucency inorganic material is superior in heat conductivity to the translucency organic material. Moreover, the use of the translucency inorganic material can improve heat resistance, light resistance, and moisture resistance, in comparison with the use of the translucency organic material.
When the second translucency material is selected to be the glass, a method of manufacturing the color conversion member may comprise preparing a mixture of the glass powder of the glass and fluorescent powder of the fluorescent material, molding the mixture into a desired shape followed by baking the same.
The above method of manufacturing the color conversion member 170 enables to provide the dense color conversion member (fluorescent material dispersed glass) 170 containing less amount of air bubbles when the mold is baked at a higher temperature. In this instance, however, there is a likelihood of lowered quantum efficiency or coloring of the glass due to the fluorescent material being oxidized or reacted with the glass. While, on the other hand, the lowering of the quantum efficiency is avoided when the mold is baked at a lower temperature, however, it is likely to lower the transmissivity of the glass due to the bubbles remaining in a higher amount within the color conversion member 170.
The decrease of the quantum efficiency of the fluorescent material and the coloring of the glass are factors to cause decrease of light output of the light emitting device 101.
Therefore, development of the color conversion member 170 which can reduce the decrease of the quantum efficiency of the fluorescent material and prevent the coloring of the glass is coveted
Moreover, in the light emitting device 101, it is difficult to thin a thickness of the color conversion member 170 because the color conversion member 170 is required to keep its dome shape. As the thickness of the color conversion layer 170 is increased, a difference in optical paths of the light emitted from the LED chip 110 toward various portions of the color conversion member 170 is also increased. As a result, color unevenness occurs easily.